Much of our general understanding of coactivator function stems from characterizations of the p160/Steroid Receptor Coactivator (SRC) family of coregulators, which comprises SRC-1, -2, and -3 full length (SRC-3FL). Our prior work revealed that coactivators are controlled by a complex posttranslational modification (PTM) code that triggers a multitude of changes to regulate their molecular functions. Such PTM-induced changes explain how a single SRC coregulates diverse transcriptional outputs that enable the execution of wide-ranging physiological and pathophysiological responses, including even nongenomic activities in the cytoplasm and cell membrane. These non-nuclear roles for coactivators have been significantly expanded through our discovery of a new SRC-3 isoform (SRC-3?4) which functions as an essential molecular adaptor for growth factor induced signaling at the plasma membrane. Because our recent data suggesting that crosstalk between SRC- 3FL (from the nucleus) and SRC-3?4 (at the plasma membrane) is critical for coordinate control of cell proliferation and motility, expansion of this concept will be a major focus of this renewal application. Finally, our preliminary data show that Gene Regulated by Estrogen in Breast -1 (GREB1 (previously known as an estrogen target)) plays a critical role in the determination of the ER- positive luminal epithelial cell type and in the response of the differentiated target cell to estrogen. Therefore, a major element of this proposal will be to determine how ER, SRC-3FL and GREB1 interact to maintain identity of the luminal epithelial cell during its normal proliferative response to estrogen and promote loss of differentiation during mammary tumorigenesis.